To make the best use of both radiation and hyperthermia as cancer treatment modalities, a more complex understanding of their cytocidal actions is important. In the proposed investigations Chinese hamster ovary (CHO) cells growing in vitro will be used to study the molecular mechanisms involves in cell killing by radiation and hyperthermia. Using DNA-incorporated radionuclides of different linear energy transfer (LET) the role of repair in the differential radiosensitivity of early verses late replicating DNA will be assessed from the standpoints of cell survival, the induction of chromosomal aberrations, and the relationship between the localization of the radiation and the chromosomal location of the aberrations. Using these same endpoints, the ways in which hyperthermic treatments alter the repair of radiation induced damage will be studied. The relationship between hyperthermia-induced chromosomal aberrations in S phase cells and the DNA undergoing synthesis at the time of heating will be determined by tritiated thymidine (3HTdR) labeling of newly forming DNA immediately prior to heat treatment. This relationship will also be studied for the aberrations produced when hyperthermia is combined with externally applied - X or -Gamma irradiation. Furthermore, modification of the effects of hyperthermia on these various endpoints by low environmental pH (eg. 6.6) or membrane active agents (eg. procaine-HC1) will be determined. The data resulting from these studies should help determine the role of chromosomal aberrations in cell killing by radiation and hyperthermia, and the importance of the inhibition of repair of DNA damage in hyperthermic radiosensitization.